Heterozygous as well as homozygous containing hemoglobin S (HbS) red cells contain higher concentrations of methemoglobin than those not containing the mutant protein. Conditions have been developed for low temperature investigations of red cells that have been incubated 37x. Oxidized hemoglobin concentrations (S = 5/2 and g imid for S = 1/2) are related to catalase levels and correlated with hematocrit of samples under investigation. Comparisons are made with normal red cells. It was found that HbS-containing cells have higher concentrations of oxidized hemoglobin than normals.